Internet-based instant messaging applications for exchanging information within selected user groups without the familiar time delays of conventional e-mail applications are growing increasingly popular. Typical instant messaging applications enable a user to prepare a message to members of a selected user group, also referred to as a “buddy list”, and to send the message for essentially immediate receipt. Messages sent by members of the selected user group in the opposite direction are received by the user analogously.
WO 02/43351 discloses a communication system in which a first server is provided to make available program applications and to exchange data with a first client, and a second server is provided to receive and to manage presence information assigned to the first client. Presence information indicates whether a user is currently obtainable at a specified terminal in the communication system. The first server is also provided in order to exchange data with a second client on the basis of the presence information assigned to the first client.
The presence information in the communication system described in WO 02/43351 can additionally be linked with a program application. If, for example, a user accesses a program application via a terminal, the program application assigns the presence information “active” to the user and monitors the activity of the user at the terminal. If no user activity is detected for a certain period of time, the program application triggers the sending of a check message to the terminal. If the user confirms the check message at the terminal, the presence information “active” continues to be assigned to the user. If the user fails to confirm the check message at the terminal, the presence information “inactive” or a similar appropriate value is assigned to the user. The presence information assigned to the user can also influence the delivery of messages from other users. Such a message is only delivered, for example, if the presence information “active” is actually assigned to the user at the time. If the presence information “active” is not assigned to the user at the time, the message is either deleted or forwarded to a different destination identified by the presence information.
It is a fundamental principle of instant messaging systems that the potential addressee of a message be obtainable. Delays in the sending and answering of messages are usual in e-mail systems, but it is generally undesirable in instant messaging systems to send a message to an addressee who will only receive the message hours or days later. Precautionary measures to determine exact presence information assigned to users in a communication system are consequently very important.